The design and construction of an exposure meter for use with infrared sensitized film

Author:

Fitz, James A.

Abstract:

In this project, an exposure meter which is sensitive
to, and calibrated for infrared radiation was designed
and constructed. An EG&G SGD-100A silicon photodiode was
chosen to use as the photodetector. This choice was based
on cost, availability, ruggedness, stability, and sensitivity.
A circuit using an operational amplifier to amplify
the output of the photodetector was used. The circuit
design allowed easy variation of the signal gain to give
a detectable meter deflection under varying lighting
conditions. Five different gains were used to allow use of
the meter under conditions ranging from outside under
direct sunlight, to inside with typical room lighting. The
gains used were empirically established.
The film used was Kodak HIE 135-20. This is a black-and-white panchromatic emulsion with special sensitizing
agents added to extend the film's sensitivity into the
near infrared to about 900 nanometers. The spectral response
of the photodetector was to be filtered to match the
spectral response of this infrared sensitized film. This
proved not to be possible, due to the unavailability of
any regular, stock filter which cuts off wavelengths above
900 nanometers. Despite this problem, the meter is much
more accurate for setting exposures for the infrared
sensitized film used than a normal exposure meter. The
designed meter typically indicates within one stop of
the optimum exposure for this film. A normal exposure
meter reading made using Kodak's exposure recommendations
was incorrect by as much as three stops.